Supported silver catalysts have long been used in the conversion of ethylene and oxygen to ethylene oxide. The use of small amounts of the alkali metals, K, Rb and Cs, were noted as useful promoters in supported silver catalysts in U.S. Pat. No. 3,962,136, issued Jun. 8, 1976 and U.S. Pat. No. 4,010,115, issued Mar. 1, 1977.
Rhenium was noted as being effective in improving selectivity of alkali metal doped silver based catalyst supported on a porous refractory support in U.S. Pat. No. 4,761,394 issued Aug. 2, 1988 and in U.S. Pat. No. 4,833,261 issued May 23, 1989. U.S. Pat. No. 4,766,105 issued Aug. 23, 1988, U.S. Pat. No. 4,820,675 issued Apr. 11, 1989, and U.S. Pat. No. 4,808,738 issued Feb. 28, 1989 further disclose the use of sulfur, Mo, W, Cr as rhenium co-promotor for such rhenium promoted catalyst.
These rhenium promoted catalysts exhibit exceptionally high selectivity as compared to conventional rhenium free catalysts. However, it exhibits lower initial activities and faster activity decline rate than that of the conventional rhenium free catalysts.
In commercial operation, reactor temperature is gradually increased to maintain an acceptable ethylene oxide production rate as catalyst activity diminishes. Ethylene oxide catalysts are generally run until the upper temperature limit of the unit is reached. Also, acceptable selectivity must be maintained throughout the life of the catalyst. Therefore, the lifespan of a catalyst depends on five factors: 1) initial activity, 2) activity decline rate, 3) upper temperature limit of the reactor, 4) initial selectivity, and 5) selectivity decline rate.
As a result of the low initial reactivity and faster activity decline rate, a reactor loaded with rhenium promoted catalyst has to be operated at a higher temperature at the initial stage and the temperature "window of operation" prior to reaching the metallurgical limits of the commercial reactor is considerably narrower. Consequently, the commercial lifespan of rhenium promoted catalyst is shorter than a conventional catalyst- The lifespan of the rhenium promoted catalyst is especially shortened in commercial reactors that have restrictive temperature limitations, most notably in the increasingly popular water cooled reactors.
Increasing initial catalytic activity as well as holding activity and selectivity stability for rhenium promoted catalyst, while maintaining the selectivity advantage, is believed to be one of the most critical issues for the development of an improved high selectivity rhenium promoted catalyst with long catalyst life.
Some references in the art have suggested the use of group IVB metals as an ingredient in the silver based ethylene oxide catalyst. U.S. Pat. No. 4,908,393 issued Mar. 13, 1990 and U.S. Pat. No. 5,057,481 issued Oct. 15, 1991 disclosed cesium promoted, silver based supported catalysts containing oxyanion of group 3b through group 7b, including, among a large group of oxyanions, titanate and zirconate.
Japanese patent application 78,012,489, published May 1, 1978, disclosed an olefin oxides silver based catalyst containing group IV element. The catalyst is not supported by a porous support and does not contain rhenium.
European patent application 266,015 published May 4, 1988 disclosed a silver based ethylene oxide catalyst containing rhenium metal promoter and at least one further metal promoters. A large number of metals, including group IVB metals, were listed as suitable further metal promoters. The metal promoters are believed to be present as oxidic compounds in numerous forms listed, including oxides, hydroxides, nitrates, sulfates, carboxylates, carbonates, bicarbonates, oxyhalides, etc,. However, there was no mention as to which oxidic form provides better catalytic performance.
It has now been found, that the addition of group IVB oxycation-containing salts to a silver based ethylene oxide catalyst, having a promoting amount of alkali metal and promoting amount of rhenium, provides substantial improvement in both initial activity and long term stability in selectivity, while the high initial selectivity advantage of prior art rhenium promoted catalysts is maintained. Furthermore, the activity advantage is maintained throughout the catalyst life time.